Handbook for Sound Engineers

774 Chapter 22

require no calculation. This network is designed to work
between impedances of equal value only. The contact
arms for resistors R 5 and R 6 are connected mechanically
by a common shaft and vary inversely in value with
respect to each other.

A balanced bridged T attenuator is a configuration
similar to the unbalanced bridged T attenuator, except
the resistor elements are divided and placed in each side
of the line, Fig. 22-12. The principal objection to the use
of this configuration, if made variable, is that the shunt
resistor R 6 must be divided into two separate arms to
provide a ground connection at the exact electrical
center. However, if the circuit feeding or terminating the
attenuator is balanced to the ground, the ground connec-
tion at the attenuator center will not be required.

The resistor values are calculated with the following
equations:

(22-31)

(22-32)

(22-33)

where,

Z is the line impedance in ohms,

R 5 is the bridging resistor in ohms,

R 6 is the shunt resistor in ohms.

The values of K and l are taken from Table 22-1.

The impedance variations for a typical high quality

attenuator used in a mixer network are shown in Fig.

22-13. The greatest impedance variation occurs as the

attenuator arm approaches zero attenuation and amounts

to about 80:. This impedance variation is not too

serious, as the mixer-combining network with its

building-out resistors isolates this variation to a great

extent from associated attenuators.

22.2.5 S or ' Attenuators

A S or ' attenuator is a resistive network resembling

the Greek letter pi (S), or delta ('), Fig. 22-14. Such

networks may be used between impedances of equal or

unequal values.

For networks operating between impedances of

equal value

(22-34)

(22-35)

where,

R 1 is the input and output resistor in ohms,

R 2 is the series resistor in ohms,

Z is the input and output impedance in ohms,

Find e and g in Table 22-1.

Figure 22-11. A bridged T attenuator. For variable pads, the
arms R 5 and R 6 are made variable.

Figure 22-12. Balanced bridged T attenuator. For a variable
configuration, variable arms are required.

p p

R 1 R 1

R 6

R 5

Z 1 Z 1

p p

p p

R 1

2

R 1

2

R 1

2

R 1

2

R 5

2

R 5

2

R 6

2

R 6

2

Z 1 Z 1

R 1 = Z

R 5 = K 1– Z

Figure 22-13. Impedance characteristics for a high quality

variable bridged T attenuator.

R 6 =Zl

p p

Input Output

700

600

500

400

300

200

100

0

Attenuation

70

60

50

40

30

20

10

0

Counterclockwise rotation, 30 steps, 1.5 dB per step

Impedance–ohms

Loss–dB

R 1 =Ze

R 2

Z

2

=--- g